4wheelers just dont have aftermarket parts like cars do.
Actually things have changed a lot over the last decade or so. You can now buy aftermarket parts,
often made in China, for the more popular ATV models. I have bought new carbs, fuel petcocks,
ignition switches, sprockets, and handlebar controls at excellent prices. Quality is pretty good, actually.
There is even a US maker of plastic fuel tanks (Clarke) that will make-to-order replacement
tanks for many ATVs. These are spendy ($200 or so), but superior to OEM steel tanks. I bought
one for an older Honda.
The very popular models, like the Honda TRX300 (1988-2000), which I also have, are covered best.
I have also recently been working on some Chinese quads, with good success. I can get electrical
parts now, directly from China, in as little as 5 days. I am hoping to substitute Chinese ignition systems
for OEM parts on Japanese ATVs. It has been a learning experience.